RISC-V: Replace simm32_p with immediate_operand (Pmode)

RISC-V: Replace simm32_p with immediate_operand (Pmode)

simm32_p is used to check constant int value within 32-bit.
It's used in handling SEW = 64 in rv32 system since such constant int
value with 32-bit allow us use vx instruction.

The current implementation of simm32_p is quite ugly and now I figure
out immedate_operand (op, pmode) can help us to check whether the op
is a constant value within 32-bit.

I already have a bunch testcases to test SEW = 64 in rv32 system and
all regression tests are passed with this patch.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (simm32_p): Remove.
	* config/riscv/riscv-v.cc (simm32_p): Ditto.
	* config/riscv/vector.md: Use immediate_operand
	instead of riscv_vector::simm32_p.